Current production needs force operations managers to assemble products in a short amount of time. However, intricate designs of products with close production tolerances are difficult to produce in such a hurried assembly. On many assemblies, a designated torque on a fastener may not be exceeded. However, if an assembly line worker is forced to use a standard torque wrench, he may have problems reading a gauge all day, especially in a hurried atmosphere. In addition, some applications may not allow the room needed for the broad, sweeping motion of a torque wrench.
In answer to this, torque-limiting tools have been introduced. These tools tighten a fastener to a given torque and then spin freely when a predetermined torque is reached. In low torque situations, a torque-limiting screwdriver may be used; that is, a tool which has a longitudinal axis that is common to the handle or driving element and the drive shaft or driven unit.
However, a problem exists in the screwdriver-type of torque tool in that torque cannot be measured as accurately as in a torque wrench because the moment arm of a screwdriver is so small. This is necessary because of design; the confined space of the screwdriver does not allow a large moment arm for measurement. In a wrench, deformation may be measured at a point one inch or more from the longitudinal axis of the fastener; in a screwdriver, the measurement must be made within about one-half inch or less. Since torque is a measurement of force times distance, the small moment arm makes the force at a given torque in the screwdriver-type tool much greater than in the wrench. Thus, small variations or deformations of measurement parts may cause large discrepancies in torque measurement. Also, large amounts of wear may occur to the parts because of the large amount of force applied. Therefore, in a torque-limiting tool, the main object is to produce a nearly frictionless slipping movement with a measuring device that will not vary in the force at which the slipping movement occurs because of wear or production imperfections. Recent low product tolerances on assembly lines demand a margin of error of four percent or less for this type of tool.
Attempts have been made to reduce wear and friction in order to produce a torque-limiting tool with a small margin of error. Examples of these types of tools are found in U.S. Pat. Nos. 2,984,133, 3,119,247, and 3,890,859. However, because of the problems discussed below, these tools do not provide a small enough margin of error.
In all of these prior devices, the torque-limiting tool comprises a drive shaft, which is the driven element, and a handle, the cylindrical driving element. The key to the torque limitation is the association between the driving element and the driven element. Generally, each element is associated with a plate which is generally perpendicular to the longitudinal axis of the tool. The two plates face each other and engage or disengage one another according to the amount of friction between the plates and the torque applied to the driving member. Thus, the two plates act much like the operation of a clutch mechanism. A spring forces the two plates together. In order to get the two plates to spin relative to one another, a torque must be applied to the driving element that overcomes the force of the spring and the friction of the plates. The geometrical configuration of the two plates determines the amount of friction and wear and therefore the accuracy of the clutch mechanism.
U.S. Pat. No. 2,984,133 teaches the use of a pair of dimple plates with balls interposed therebetween. When torque is applied to the dimple plate associated with the driving element, a moment arm is created across the ball. This causes the ball to apply a large amount of pressure on the edges of the dimple where the ball meets the surface of the plate, which creates a high wear area. Once this area is worn, duplication of engagement of the two plates is hard if not impossible to keep. In addition, the balls may roll out of the pocket.
U.S. Pat. No. 3,890,859 teaches a clutching assembly using balls and cylinders in conjunction with a ball driving bar. The cylinders are halfway inset into the first plate. These cylinders cross each other at the longitudinal axis of the torque-limiting tool and therefore are not rotatable. The opposing plate has a ball-engaging drive bar and balls that are interposed between the cylinders and the drive bar. When a proper amount of torque is applied, the drive bar forces the balls to engage the cylinders and eventually "roll" over them. Friction between the ball and the drive bar, and friction between the ball and the first plate cause inaccurate measurements in this torque-limiting tool. In addition, the cylinders, because they are not rotatable, have a constant point of contact which wears quickly. Thus, the instrument loses calibration after a number of cycles.
U.S. Pat. No. 3,119,247 also uses balls and cylinders in its clutching mechanism. However, in this invention, the balls are seated between a longitudinal guideway in the cylindrical driving member and concave seats in a plate positioned perpendicular to the longitudinal axis of the driving member. The seats force the balls up against the groove. The balls are allowed free movement longitudinally but not radially in the driving member. A spring pushes the balls toward a cylinder which passes through and is perpendicular to the driven unit. The cylinder may or may not be free to rotate. This invention still presents problems, however. The balls encounter friction at the guideway, the concave seats of the plate, and on the surface of the cylinder. Although the cylinder is free to rotate, it encounters two balls at once which work in opposite directions of rotation on the cylinder, preventing rotation of the cylinder. Thus, the movement of the balls in the guideway and the inability of the cylinder to rotate in response to force applied from the balls causes the device to have an excessive margin of error.
Thus, there is a need to restrict friction between the movable parts of the clutching mechanism of torque-limiting tools. This necessity dictates limiting points of contact between the opposing friction members to a minimum. However, these few fixed coordinates of contact need to be defined by varying surface locations on the engaging metal parts in order to reduce wear and therefore increase accuracy of calibration.